In-situ cementation for profile control

ABSTRACT

A method and composition for controlling the profile of a formation where temperatures higher than 200° F. are encountered. An aqueous solution of an organoammonium silicate, alkali metal or ammonium silicate is injected into a zone of higher permeability in a formation. Subsequently, a spacer volume of a water-immiscible organic solvent is directed into said zone. Afterwards, a water-miscible organic solvent containing an alkylpolysilicate is injected into the higher permeability zone. A silica cement is formed in-situ thereby substantially closing the higher permeability zone to fluid flow. Thereafter, a steam-flooding, water-flooding, carbon dioxide-flooding, or fire-flooding EOR operation is commenced in a lower permeability zone.

This is a division of copending application Ser. No. 07/810,586, filedon Dec. 19, 1991 now U.S. Pat. No. 5,211,231.

FIELD OF THE INVENTION

This invention relates to the plugging of a more permeable zone of asubterranean formation. More particularly, the invention relates to anovel method of forming a silica cement in-situ which covers asubstantial areal extent of a more permeable zone. This method isespecially useful in promoting more uniform fluid injection patterns soas to tolerate neutral or high temperature steam while conducting asteam-flooding, water-flooding, carbon dioxide-flooding, orfire-flooding enhanced oil recovery operation (EOR) in a lesserpermeability zone.

BACKGROUND OF THE INVENTION

Steam or fire stimulation recovery techniques are used to increaseproduction from an oil-bearing formation. In steam stimulationtechniques, steam is used to heat a section of a formation adjacent to awellbore so that production rates are increased through lowered oilviscosities.

In a typical conventional steam stimulation injection cycle, steam isinjected into a desired section of a reservoir or formation. A shut-in(or soak phase) may follow, in which thermal energy diffuses through theformation. A production phase follows in which oil is produced until oilproduction rates decrease to an uneconomical amount. Subsequentinjection cycles are often used to increase recovery.

Steam stimulation techniques recover oil at rates as high as 80-85% ofthe original oil in place in zones at which steam contacts thereservoir. However, there are problems in contacting all zones of aformation due to heterogeneities in the reservoir, such as high/lowpermeability streaks, which may cause steam fingering. When any of theseheterogeneities are present in a reservoir, the efficiency of a processbegins to deteriorate due to reduced reservoir pressure, reservoirreheating, longer production cycles and reduced oil-steam ratios. As aresult, steam stimulation may become unprofitable.

Various methods have been proposed so that steam can be diverted touncontacted zones of a formation. One such method is disclosed in U.S.Pat. No. 2,402,588 which issued to Andersen. Andersen disclosed a methodof filling a more permeable zone of a reservoir by injecting a dilutealkaline solution of sodium silicate under low pressure. An acid gassuch as carbon dioxide is then injected to reduce the alkalinity of thesolution, which results in the forming of a silica gel.

Another method is disclosed in U.S. Pat. No. 3,645,446 which issued toYoung et al. Young discloses the plugging of a zone of a reservoir byinjecting a mixture of steam and sodium silicate into the permeablezone. A second mixture containing steam and a gelling agent such ascarbon dioxide is injected into the permeable zone and the two mixturesare allowed to react. A hard silica gel plug is formed.

Another method is disclosed in U.S. Pat. No. 3,805,893 which issued toSarem. Sarem discloses the formation of a gelatinous precipitate byinjecting small slugs of a dilute aqueous alkali metal silicatesolution, followed by water and then a dilute aqueous solution of awater-soluble material which reacts with the alkali metal silicate toform a precipitate. The precipitate hardens to form a substantiallyimpermeable substance. A water-flooding oil recovery method is thenconducted in a lower permeability zone.

Christopher discloses another method in U.S. Pat. No. 3,965,986. In thismethod, a slug of liquid colloidal silica and water is injected into areservoir. This slug has a relatively low viscosity. A surfactant isnext injected therein which forms a gel on contact with the silica slug.

Amino resins such as melamine formaldehyde resins are cross-linked withcertain polymers to make gels useful as profile control agents for hightemperature reservoirs during a water-flooding operation. These gels aredisclosed in U.S. Pat. No. 4,834,180 which issued to Shu on May 30,1989. These gels are unable to withstand high temperatures encounteredduring a fire-flooding enhanced oil recovery operation.

Therefore, what is needed is a method for consolidating a highpermeability zone of a formation while controlling the permeability ofthat zone with a natural silica cementing material so as to enable theconducting of an EOR method such as a steam-flooding, carbondioxide-flooding, water-flooding or fire-flooding operation in a zone oflesser permeability where high temperatures and pH's of 7.0 or less areencountered.

SUMMARY OF THE INVENTION

This invention is directed to a method for permeability profile controlwhere a steam-flooding, water-flooding, carbon dioxide-flooding, orfire-flooding EOR operation is utilized or where high temperatures andpH's of 7.0 or less are encountered. In the practice of this invention,an aqueous organoammonium silicate, alkali metal or ammonium silicatesolution is injected into a higher permeability zone of the formation.The silicate is prevented from entering a zone of lower permeability byutilization of a mechanical packer. As the aqueous organoammonium,alkali metal or ammonium silicate enters the higher permeability zone,it saturates said zone.

Thereafter, a spacer volume of a water-miscible organic solvent isdirected into the higher permeability zone. The solvent is selected froma member of the group consisting of methanol, ethanol, higher alcohols,ketones, tetrahydrofuran, and dimethyl sulfoxide.

After the spacer volume of solvent has been placed into the higherpermeability zone, a water-miscible organic solvent containing analkylpolysilicate is next injected into the higher permeability zone.Upon coming into contact with the organoammonium silicate, alkali metalor ammonium silicate solution which has saturated the higherpermeability zone, alkylpolysilicate reacts with the organoammoniumsilicate, alkali metal or ammonium silicate to form a silica cementin-situ. The silica cement which is formed is stable at neutral or lowpH's and temperatures in excess of about 200° F. These above steps canbe repeated until the zone has been closed or consolidated to the extentdesired. Thereafter, a steam stimulation, carbon dioxide,water-flooding, or fire-flooding EOR operation is initiated in a zone oflower permeability in said formation to remove hydrocarbonaceous fluidstherefrom.

By controlling the strength and rate of injection of the organoammoniumsilicate, alkali metal or ammonium silicate and the alkylpolysilicatewhich are injected into the higher permeability zone, the higherpermeability zone of the formation can be closed to fluid flow.

It is therefore an object of this invention to provide for a method ofmaking a silica cement in-situ for controllably plugging or closing ahigher permeability zone within a formation which cement is more naturalto a formation's environment.

It is even another object of this invention to consolidate a looselyconsolidated zone in a formation while controlling the profile of thatzone.

It is another object of this invention to provide for a compositionwhich will ensure an even flow front and a homogeneous consolidationwhen closing a higher permeability zone where neutral and low pH's orhigh temperatures are encountered.

It is a further object of this invention to provide for a simple processfor blocking a zone which tolerates residual oil and formation brinewhile making a silica cement in-situ while controlling the distributionand setting of said cement.

It is a still yet further object of this invention to provide for aformation profile control method which can be reversed by treating theconsolidated interval with an alkali solution.

It is an even still yet further object of this invention to provide fora formation consolidation agent which is resistant to high temperaturesand neutral or low pH's.

It is a yet even still further object of this invention to provide for asimple process for closing a higher permeability zone in a formationwhich avoids complex procedures of clearing and removing water from saidzone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration which shows placement of thecomponents of this invention in a higher permeability zone of aformation.

FIG. 2 is a schematic illustration which depicts a higher permeabilityzone closed with the composition of this invention while an enhanced oilrecovery (EOR) method is being initiated in a lower permeability zone.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention concerns an improvement in the sweep efficiency of an EORprocess by injecting sequentially solutions sufficient to form a silicacement in-situ in a higher permeability zone of a formation so as toclose an oil-depleted zone. When a need to close the higher permeabilityzone has been determined, an aqueous solution containing anorganoammonium silicate, alkali metal or ammonium silicate is injectedinto the higher permeability zone. Once the aqueous silicate solutionhas progressed into the higher permeability zone to the extent desired,a spacer volume of a water-miscible organic solvent is next injectedinto the higher permeability zone to separate the aqueous silicate slugfrom an organic solvent containing an alkylpolysilicate.

This spacer volume of solvent is selected from a member of the groupconsisting of methanol, ethanol, higher alcohols, ketones,tetrahydrofuran, and dimethyl sulfoxide. Solvents used as a spacervolume may be of an industrial grade. Utilization of the spacer volumeshould be kept to a minimum in order to obtain a higher degree ofplugging. If residual permeability is desired the volume of solvent slugshould be increased.

After placing the spacer volume of solvent into the higher permeabilityzone, a solvent containing an alkylpolysilicate is directed into thehigher permeability zone. This alkylpolysilicate reacts with theorganoammonium silicate, alkali metal or ammonium silicate therebyforming a silica cement in-situ which consolidates and controlspermeability in the higher permeability zone. Injection of theorganoammonium silicate, alkali metal ammonium silicate, spacer volumeof solvent, and alkylpolysilicate can be repeated until the higherpermeability zone is consolidated and its permeability reduced to adesired extent.

In order to increase the cement's consolidating and plugging ability,the concentration of the organoammonium silicate, alkali metal silicateor ammonium silicate contained in an aqueous solution or thealkypolysilicate contained in the solvent can be increased. Similarly,the flow rates of each of these solutions through the higherpermeability zone can be decreased to obtain better consolidation andplugging strength. A decreased flow rate is particularly beneficial forincreasing plugging or consolidation and controlling permeability whenthe alkylpolysilicate solution's flow rate is decreased. As will beunderstood by those skilled in the art, optimal concentrations and flowrates are formation dependent. Therefore, optimal concentrations andflow rates should be geared to field conditions and requirements.

Organoammonium silicates which can be used in an aqueous solutioninclude tetraammonium silicate and other alkyl, aryl, or hetero atomscontaining moieties such as sulfur or oxygen and ten or less carbonatoms are preferred.

After the higher permeability zone has been consolidated and thepermeability reduced to the extent desired, a thermal oil recovery orwater-flooding EOR method is initiated into a zone of lesserpermeability within the formation and hydrocarbonaceous fluids areremoved therefrom. The formations which are plugged and consolidated caninclude unconsolidated or loosely consolidated ones. Unconsolidated sandformations are also included. While the EOR method is being conducted inthe lower permeability zone, temperatures within that zone may exceedabout 400° F. This zone may also have a pH of 7 or less. A method forthe selective placement of polymer gels for profile control in a thermaloil recovery method is discussed in U.S. Pat. No. 4,804,043 which issuedto Shu et al. on Feb. 14, 1989. This patent is hereby incorporated byreference herein.

Steam-flooding processes which can be utilized when employing thisprofile control method described herein are detailed in U.S. Pat. Nos.4,489,783 and 3,918,521 which issued to Shu and Snavely, respectively.U.S. Pat. No. 4,479,894 that issued to Chen et al. describes awater-flooding process that can be used herein. Fire-flooding processeswhich can be utilized herein are disclosed in U.S. Pat. Nos. 4,440,227and 4,669,542 which issued to Holmes and Venkatesan, respectively. Thesepatents are hereby incorporated by reference herein.

A carbon dioxide EOR process which can be used after consolidating thehigher permeability zone is disclosed in U.S. Pat. No. 4,513,821 whichissued to W. R. Shu on Apr. 30, 1985. This patent is hereby incorporatedby reference herein.

Organoammonium silicate, ammonium or alkali metal silicates having aSiO₂ /M₂ O molar ratio of about 0.5 to about 4 are suitable for formingan acid stable silica cement. The metal (M) which is utilized hereincomprises sodium, potassium, or lithium ions. Sodium and potassiumsilicate comprise the preferred inorganic silicates. Silicatescontaining ammonium ions can be used also. Preferably, the SiO₂ /M₂ Omolar ratio is in the range of about 0.5 to about 4 or greater. Theconcentration of the silicate solution is about 10 to about 60 wt.percent, preferably 20 to about 50 wt. percent. As will be understood bythose skilled in the art, the exact concentration should be determinedfor each application. In general, concentrated silicate solution aremore viscous and more effective in plugging or consolidation due tohigher contents of solids.

The viscosity of the silicate solution can also determine the extent towhich it will enter a higher permeability zone. In those cases where itis not possible to control the viscosity of the silicate solution andpreclude entry into a lower permeability zone, a mechanical packer maybe used. The silica cement which is formed can withstand pH's less thanabout 7 and temperatures up to and in excess of about 200° F. Thepreferred silicates are sodium and potassium. Potassium is preferredover sodium silicate because of its lower viscosity. Fumed silica,colloidal silica, or alkali metal hydroxides can be added to modify theSiO₂ /M₂ O molar ratio of commercial silicate. Colloidal silicate can beused alone or suspended in the alkali metal silicate as a means ofmodifying silica content and pH.

Alkylpolysilicate is the hydrolysis-condensation product ofalkylorthosilicate according to the reaction equation below: ##STR1##where n≦2

R=C₁ -C₁₀

R should be ≦10 carbons for good solubility and high SiO₂ content.

Tetramethyl or tetraethylorthosilicates (EPS) are preferred. Mixedalkylorthosilicate can also be used. It is desirable to obtain analkylpolysilicate with n>0.5, preferably greater than about 1. As nincreases, the SiO₂ content increases, resulting in strongerconsolidation. It is desirable to use an alkylpolysilicate with a silicacontent of 30% or more, preferably about 50% wt. percent. EPS which areused herein are placed into one of the organic solvents mentioned above.The preferred solvent is ethanol. Of course, other alcohols can be used.EPS or other alkylpolysilicates are contained in the solvent in anamount of from about 10 to about 90 weight percent sufficient to reactwith the silicates contained in the aqueous solution. Although alcoholis the solvent preferred because of its versatility and availability,other water-miscible organic compounds can be utilized. These solventsinclude methanol and higher alcohols, ketones, tetrahydrofuran, anddimethyl sulfoxide.

Referring to FIG. 1, an aqueous solution of an organoammonium silicate,alkali metal or ammonium silicate is injected into injector well whereit enters high permeability zone 14 of reservoir 20 through perforations18. Next a spacer volume of an organic solvent is injected into zone 14.Afterwards, a solvent containing an alkylpolysilicate therein isinjected into higher permeability zone 14 where it forms in-situ asilica cement which is stable to temperatures up to and in excess ofabout 200° F. Once the silica cement has hardened and higherpermeability zone 14 has been plugged and consolidated to the extentdesired, by repeated applications if necessary, an EOR operation isinitiated into lower permeability zone 16 as is shown in FIG. 2. Aflooding medium used in the EOR operation exists reservoir 20 byperforations 18 into producer well 12.

As the aqueous organoammonium silicate, alkali metal or ammoniumsilicate solution proceeds through higher permeability zone 14, itdeposits a film of said aqueous silicate on sand grains therein. Thisaqueous silicate also fills intersitial spaces between the sand grains.A spacer volume of the solvent is directed through zone 14 so as toseparate aqueous silicate from the alkylpolysilicate contained in thesolvent. The solvent containing the alkylpolysilicate is injected intozone 14 in a concentration and at a rate sufficient to bind andconsolidate the sands in zone 14 thereby closing or plugging said zoneby forming a silica cement therein.

Although the present invention has been described with preferredembodiments, it is to be understood that modifications and variationsmay be resorted to without departing from the spirit and scope of thisinvention, as those skilled in the art readily understand. Suchvariations and modifications are considered to be within the purview andscope of the appended claims.

What is claimed is:
 1. A silica cement sufficient to close a zone of aformation to fluid flow obtained by a process comprising the steps of:a)injecting into said zone an aqueous solution of a silicate selected froma member of the group consisting of an alkali metal silicate, ammoniumsilicate and organoammonium silicate; b) injecting next into said zone aspacer volume of a water-miscible organic solvent selected from thegroup consisting of methanol, ethanol, higher alcohols, ketones,tetrahydrofuran, and dimethyl sulfoxide; and c) injecting thereafterinto said zone a water-miscible organic solvent containing analkylpolysilicate in an amount sufficient to react with the silicatesolution of step a) when the solutions of steps a) and b) are contactedwith a zone of a formation so as to form a silica cement in-situ.
 2. Thecement as recited in claim 1 where the alkali metal silicate comprisesions of sodium potassium, or lithium, and mixtures thereof.
 3. Thecement as recited in claim 1 where the alkali metal silicate has asilicon dioxide to metal oxide molar ratio of about 0.5 to about
 4. 4.The cement as recited in claim 1 where alkali metal silicate,organoammonium silicate, or ammonium silicate is contained in thesolution in an amount of from about 10 to about 60 weight percent. 5.The cement as recited in claim 1 where in step c) the alkylpolysilicateis contained in said solvent in an amount of about 10 to about 90 weightpercent.
 6. The cement as recited in claim 1 where said silica cementwithstands temperatures in excess of 200° F.
 7. The cement as recited inclaim 1 where the silica cement withstands a temperature in excess of200° F. and a pH equal to or less than about
 7. 8. The cement as recitedin claim 1 where the alkali metal silicate has a silicon dioxide tometal oxide molar ratio greater than about
 2. 9. The composition asrecited in claim 1 where in step c) said alkylpolysilicate is ahydrolysis-condensation product of alkylorthosilicate according to theequation below: ##STR2## where n≦2 and R=C₁ -C₁₀.
 10. The cement asrecited in claim 1 where the alkylpolysilicate has a silicon content ofat least about 30 wt. percent.
 11. The cement as recited in claim 1where said organoammonium silicate comprises C₁ through C₁₀ alkylgroups, aryl groups and hetero atom containing moieties.